Gas chromatography is used for separating and analyzing compounds that can be vaporized without decomposition. Gas chromatography may be used to test compound purity, separate different compounds in a blend, or aid in compound identification.
A gas chromatograph has a mobile or moving phase which is an inert carrier gas. The gas chromatograph also has a stationary phase which is a material on an inert solid support inside a tube or column. The compound(s) in or components of a sample being analyzed or separated are injected into the chromatograph system, converted into a gas (if not already a gas), and transported to the column or tube where the analyte interacts with the stationary phase in the column. The compound(s) in the injected sample flow through the column with a carrier gas and are eluted at different times as a function as how they are retained by the stationary phase in the column. A comparison of retention times, which are a function of the physical properties of the column and components of the sample, provide a way to analytically identify and separate compounds or components in the sample. Generally substances are qualitatively identified by the order in which they emerge (elute) from the column and by the retention time of the component in the column. As the separated volatilized chemical components exit the end of the column, they are detected with a sensor/detector and identified. In addition to the physical properties of the column and the compound in the sample, parameter such as carrier gas, flow rate, column length, and temperature of the various components of the chromatographic system can also affect the retention time of the eluates.
There can be a wide range of carrier gases. These gases can include hydrogen and helium. Helium is non-flammable and works with a great number of sensors/detectors and as a result is one of the most common carrier gases used.
Technology sometimes referred to as lab-on-a-chip also exists for the separation and analysis of very small amounts of gases and/or liquids. Minute amounts of gases or liquids may be separated and analyzed by putting the gas or liquid through microchip channels having diameters of from about 10 μm to 100 μm. In the channels, retention times of different components of a sample are not the same because of a difference in affinities to a stationary phase.
Separation of very small samples into their minute components, detection, identification of, and determination of the relative amounts of components in extremely small liquid or gas samples and/or components is a challenging problem.
Gas chromatography systems use sensors to determine when a gaseous component of a sample is exiting the column. Common sensors include flame ionization sensors, thermal conductivity sensors, and mass spectrometers. Thermal conductivity sensors can be used to detect components other than the carrier gas when their thermal conductivities are different from that of the carrier gas at the operation temperatures of the gas chromatograph. Flame ionization sensors generally are more sensitive to hydrocarbons than thermal conductivity sensors but cannot detect water or other completely oxidized molecules. They require a supply of flammable gas (usually hydrogen). Mass spectrometers detect the ionic masses of the product after ionization and are complex and large.
There is always a need for sensors and detectors that include the latter sensors with higher sensitivity and sensitivity to a broad range of compounds. In addition, simplicity of design and operation, economy of operation in terms of cost and materials, small sensor size, and the simplicity of integrating the sensor into field and process instrumentation are important. This is particularly the case for MEMS devices used for separation in volumes as low as a picoliter or less.
Full sized gas chromatographs and implementation of separation and detection devices on MEMS devices can provide data in the form of graphs or “chromatograms” of sensor/detector responses (generally the y-axis) against retention time (generally the x-axis). Chromatographs and MEMS devices provide a spectrum of peaks representing the components in a sample moving through and exiting from the column or chip at different times. The time a component of a sample is retained in a column or chip can be used to identify the component when conditions are constant. Areas under the peaks are proportional to the amounts of sample components moving through the chromatograph or chip; hence, using these areas permit the determination of relative amounts of components in the sample. The concentration of a component in a sample can be determined by measuring the response for a series of concentrations of the component of interest or by determining the relative response factor of the component which the expected ratio of a component to a standard put through the chromatograph. The response factor is calculated by determining the response of a known amount of component and a constant amount of chemical added to the sample at a constant concentration that has a distinct retention time relative to the component of interest.
The ultimate goal of any chromatographic separation, microchip separation using a MEMS device, or spectrographic method and analysis is to achieve the greatest level of sensitivity that is possible to determine if and what amounts of each components of a sample are present. There are three principal advantages of using the proposed sensor in chromatography. First, the two dimensional sensors (TDS) described herein are much more sensitive to at least some eluting molecules, even below a pico gram (pg) limit. Second, the electrical and materials requirement of the TDS are very low; a few microamp source or a constant voltage source are all that is required beyond those for the chromatographic system itself. This is important for field measurements where easy access to electricity and/or gases may be limited. Third, the size of the two dimensional sensor, which includes the two dimensional material or substantially two dimensional material and/or detector that includes the two dimensional sensor, may be on a micrometer scale. TDS can be 10 μm2 to 100 μm2 on a side. Thus, TDSs are inherently good matches to MEMS scale chromatographs. Various detectors and their detection limits are described in the table below.
Type of DetectorApplicable SamplesDetection LimitMass Spectrometer (MS)Tunable for any.25 to 100 pgsampleFlame Ionization (FID)Hydrocarbons1 pg/sThermal ConductivityUniversal500 pg/ml(TCD)Electron-Capture (ECD)Halogenated5 fg/shydrocarbonsAtomic Emission (AED)Element-selective1 pgChemiluminescence (CS)Oxidizing reagentDark current of PMTPhotoionization (PID)Vapor and gaseous.002 to .02 μg/LCompounds
An ultimate goal with the chemical sensors described herein is to determine even if one atom or molecule and/or 10−21 grams of a sample are present as a component of a multicomponent sample under analysis. Such sensitivities have been demonstrated, but not in conjunction with chemical separation systems such as gas chromatographs. This is not currently possible with known chromatograph sensors. Two dimensional and substantially two dimensional materials such as graphene, graphene oxide (also known as graphite oxide), two dimensional boron nitride (BN) or boron nitride nanotubes, and two dimensional molybdenum disulfide (MoS2) can provide a detectable change in electron transport properties, such as a change in electrical resistance or the frequency of a surface oscillation, without or with very low intrinsic noise which permits measurement of the change(s) in resistance or other property effected by adsorption of 10−21 grams of elute and by even one molecule on the surface of a sensor including the two dimensional or substantially two dimensional material as described herein.
Schedin et al., in Nature Materials 6, 652-655 (2007) describe detection of individual gas molecules adsorbed on graphene measuring Hall resistivity by a 10 Tesla magnetic field. The Schedin article describes the detection of an individual gas and the determination of the concentration of the gas by reference to a standard. Simple resistance measurements have demonstrated detectivity of 10 ppb. However, no publication discusses coupling the detector with a separation device such as a gas chromatograph. Separation, detection and identification of a plurality of very small individual components in a multi-component sample by a two dimensional or substantially two dimensional material in a stationary phase and/or sensor is not described or suggested.
At present gas chromatograph systems or MEMS devices are not known that separate components of a sample and then utilize a sensor and/or stationary phase for separation where the system and devices include sensors and stationary phases that utilize two dimensional or substantially two dimensional materials. The approach described herein provides a detection device whose sensitivity matches or exceeds those in general use, is compact, is energy efficient, and integrates with MEMS scale separation and detection devices. The approaches described herein are novel because they have the potential to greatly increase the sensitivity of gas chromatography to detect individual gas molecules (1) using simple resistivity measurements such as Hall resistivity measurements, (2) using surface acoustic wave sensors which include graphene (or other two dimensional or substantially two dimensional materials as described herein), and (3) using nano-mechanical resonator measuring devices in combination with the two dimensional or substantially two dimensional materials. All of the latter are also easily integratable with MEMS scale separation devices. The applications for the devices described herein are far reaching. Small devices can be used in well drilling, exploration probes, sensing environments in inconvenient places, separating and identifying low levels of deleterious substances in mixtures of gases (such as separation and identification with gas masks), and other analytical (qualitatively and quantitative) and biomedical applications.